MDL-65621 block_myoverview: tag courses as "hidden from students"

This is intended to visually separate hidden course from open
courses in the course overview block
by applying a "Hidden from students" tag to the various views in
this block.
To achive this, the course summary exporter's output must be
augmented with a "visiblity" attribute,
carrying the given course's corresponding visibility setting.
Since the exporter now expects this data point as part of its input,
the output of the course_get_recent_courses()
function had to also be augmented with the course visibility setting
to provide this value.

course/classes/external/course_summary_exporter.php

@@ -105,6 +105,9 @@ class course_summary_exporter extends \core\external\exporter {
             ),
             'enddate' => array(
                 'type' => PARAM_INT,
+            ),
+            'visible' => array(
+                'type' => PARAM_BOOL,
             )
         );
     }